Oil to natural gas conversion - seeking advice - 1950 Cape

Converting from oil to natural gas heat & hot water from old 1950 American Standard furnace with tankless coil.
Cape style 1950 house located in northeast (Stamford, CT area) with original insulation and full unfinished basement.

Attached Files:

Getting a reasonable heat load calculation is the first and more critical part to getting it right. Anybody quoting on it WITHOUT running the heat load calc first is pretty much a hack, to be avoided.

When running a heat load calculation, don't put any thumbs on the scale such as unrealistically low outdoor design temps or high indoor temps. If if you like to keep it to 75F and it's been -10F at least once in Stamford since the last ice age, use 70-72F at most, and the 99% outside design temp (which for Stamford CT is about 14-15F.)

Let's take a rough-cut first stab at it, which is probably close enough to question any body else's calc that comes in higher:

Assuming the 1950s construction was 2x4s 16" o.c. with R11 rock-wool or fiberglass the U-factor on the walls is about 0.11 BTU per square foot per degree-F after the thermal bridging of the framing is factored in. Assuming +15F & 70F as the outdoor/indoor design temps, that's a 55F delta, so your wall losses are about:

0.11 x 55F x 1520'= 9196 BTU/hr

Assuming you have clear glass (not low-E) storm windows over wood sash double hungs the window U-factor is about 0.50 and the window losses are about:

0.50 x 55F x 225' = 6188 BTU/hr.

Lets' assume you have at least R19 in the ~ 608' of attic on 2x6 16"o.c. joists, which is a U-factor of about 0.075 after thermal bridging, so the ceiling losses are about:

U0.075 x 55F x 608' = 2508 BTU/hr

Assuming an uninsulated poured concrete foundation you have about a U-1 on the above-grade portion of the foundation. You have a perimeter of about 100', and assuming 4' of above ground exposure (counting down to a foot below grade to account for the ground losses) that would be ~400 square feet. And when it's +15F outside it's probably only 60F in the basement so your looking at a 45F not a 55F delta, making your basement losses:

U1 x 45F x 400= 4500 BTU/hr.

Figure another ~45' of U-0.5 exterior door for another:

U0.5 x 55F x 45'= 1238 BTU/hr.

Add it all up you get 23,630 BTU/hr.

If the place is pretty drafty add a 25% fudge-factor for infiltration and you're at (1.25 x 23,630=) 29,537 BTU/hr, call it 30 KBTU/hr

If the place is pretty tight it's probably more like 25K.

A modulating condensing boiler that has any more than 15K of output at minimum-fire is a model to be avoided, since even at your AVERAGE mid-winter outdoor temp it would be cycling rather than modulating. (There's simply no-point to a modulating boiler unless it's modulating MOST of the time in winter!)

The Baxi Luna Duo Tec series Model 40GA has a min-fire of 18K- a bit more than you really want, but maybe.

The TT Prestige Excellence has an even higher min-fire of 30K which makes it even worse- forget about it. (The Prestige Trimax Solo 60 dials back to 16K-in 15K out, which is about right.)

The Navien CH-180-ASME-NG (smallest of the series) pulls down to 17K in/16K out, which is probably going to be OK. The bigger versions have more hot water heating output, but also a higher min-fire. This is the one, if you go with that series.

But if you really have that much uninsulated basement it may be better to go with something cheaper a tiny forced draft cast iron boiler or a combi-tank HW heater and spend the difference in upfront cash on insulating the foundation walls and air sealing the whole house, which would likely bring your load to under 20K. When heat loads are under 20K there's no way you can get much modulating, and depending on the fin size on those convectors it's likely that you can do just fine even at +10F outdoor temps with 140F water, making a condensing tank-type water heater solution pretty reasonable.

So, what are the depth & height of the fins on those fin-tube convectors?

I have not heard back from contractor who I initially provided measurements to. Another I spoke to on the phone who I also provided measurements to is still working on it but mentioned that maybe Baxi Luna Duo Tec 1.33 GA may be an option. He wants sq footage of each room first.

The Baxi Luna Duo Tec 1.33 GA has the same min-fire ratings as the -40GA, but a lower high-fire output, but need an indirect tank for the domestic hot water.

Square footage of the room doesn't matter AT ALL! It's the square footage and U-factors of the exterior surfaces. Contractors that use a crummy rule of thumb such as "Lessee, ya got 1475 feet times 25 BTU a foot gives ya 'bout 37K, round up to 50K just to be sure..." are real hacks, always oversizing the equipment, often to the detriment of both comfort & efficiency.

With your better foundation description your basement losses are probably between 10,000-12,000 BTU/hr, which brings your heat load up to about 28-30K + infiltration. You'll note that the basement losses are by far the biggest single number (between 20% & 35% of the likely total, despite the lower presumed basement temp!). That's due to the lack of insulation. It's worth touching up the mortar, then applying 2" of closed cell polyurethane to both the foundation & band joist, painted with an intumescent paint for fire protection. That would bring the basement losses down by a full order of magnitude, cutting your heating fuel use by at least 20%, and keeping the basement both warmer & drier.

I don't have a good reference source for the output of finned convectors of 2" x 7.5" dimensions, it's probabably comparable to the 8" deep Modine convectors (see p11.) but my WAG is that they'll run on the order of 2800 BTU/foot or better @ 190F average water temp, 1400 BTU/ft @ 140F AWT, 700 BTU/ft @ 120F AWT. With 44' and a heat load of 30,000 BTU / hr you're at 680 BTU/ft, which means that you'd need maybe 130F boiler output on design-day (130 F out, 110F back), which is PERFECT for using condensing burners. (It's all one zone? Or is it two?)

It's one zone.
Maybe down the road a couple years add a zone for small heater/radiator to take chill out of basement for work area only (not to be finished).

"Square footage of the room doesn't matter AT ALL! "
Sadly, this contractor seemed the most knowledgable (although, now that I think of it, he mentioned that the Prestige Excellence might be too small).

I went back and checked fin measurements (only this time on another radiator) and discovered that this one is 5.25 x 2. I will have to go around house and confirm others. I'll get back later with this info.

Here are the updated and more accurate fin measurements after taking all covers off and measuring exact length of fins rather than length of radiator from pipe to pipe:
7'4" length of fins that are 7.25"x2" (not 7.5"). These wider ones are installed under 2 bay windows.
33'3" length of fins that are 5.25"x2"
Total Length: 40'7" (not 44')

Thanks Dana. Sounds like finding the right contractor may be harder than finding the right equipment. No one even inquired about fins. Neither of the two I provided measurements to has responded so I'm still waiting. All others simply quoted based on total sq. footage of house and looking at the 1950 Furnace.

"Still good-to go for getting good condensing efficiency with condensing boilers if your heat load is under 30K."
Anything I should be careful of on this point?

"A modulating condensing boiler that has any more than 15K of output at minimum-fire is a model to be avoided, since even at your AVERAGE mid-winter outdoor temp it would be cycling rather than modulating. (There's simply no-point to a modulating boiler unless it's modulating MOST of the time in winter!)

The Baxi Luna Duo Tec series Model 40GA has a min-fire of 18K- a bit more than you really want, but maybe.

The TT Prestige Excellence has an even higher min-fire of 30K which makes it even worse- forget about it. (The Prestige Trimax Solo 60 dials back to 16K-in 15K out, which is about right.)

The Navien CH-180-ASME-NG (smallest of the series) pulls down to 17K in/16K out, which is probably going to be OK. The bigger versions have more hot water heating output, but also a higher min-fire. This is the one, if you go with that series".

If you had to pick two modulating condenser boilers (even if not from above), which two would be at top of your list for my needs?
I have seen mixed comments about Navien brand.
If one is the Prestige Trimax Solo 60, what would you specifically choose for hot water (house has 1 and 1/2 baths)?

Don't sweat the condensing efficiency issues with the fin-tube, especially if you buy a boiler with an "outdoor reset" function.

There are many good mod-con boilers out there, but the 5:1 turn down ratio of Lochinvar Knight WH-055 lets it modulate down to ~10,000 BTU/hr, which means that even if you tighten up and insulate the place to where you're heat load is in the 20K range it can be set up to modulate most of the time, yet it still has 50K of output for reasonably fast recovery from hot water draws.

The Peerless PF-50, Triangle Tube Solo-60 Burnham Alpine-080B and several others will modulate down to ~15,000 BTU/hr, with stainless steel heat exchangers, good reputations, and at least some distributor support in New England. (In general condensing boilers with stainless heat exchangers have fewer issues than those with aluminum heat exchangers.)

With most of the smaller boilers it's better to set up the domestic hot water as a indirect-fired tank operated as a "priority" zone, since a full-flow shower+ any other hot water load would require a bigger burner, which generally means a higher low-fire. With a 1-1/2 baths sizing the indirect for the biggest tub you have to fill makes the most sense. (~40 gallons would be fine for most.)

Navien is a trademarked brand for Kyung Dong Boiler of Korea. They have been around for quite awhile, and have some good designs, but between limited US technical support and less than stellar quality control they've been given a worse rap than they actually deserve. If you go with Navien, make sure that the installer has been certified by the manufacturer, and that it's not their first combi- install. Installing modulating condensing boilers doesn't take rocket science, but it's more than just a plumbing exercise.

Installer competence is important for getting the efficiency and longevity out of any mod-con, but regional distributor technical support is useful too. If you're looking at a particular boiler, go to the manufacturer's website and find out who the local distributor is, then call the distributor for recommendations for an installer. The distributor knows more than anyone else who has been installing them by the dozen without a hitch, and who has been leaning on them to answer dead obvious "it's in the manual-stupid!" kinds of questions. With the name of a local contractor from the "not a PITA" list the incompetence risk is lower.

Thanks very much Badger. "HTP Contender or the Triangle Tube Prestige, at 50 and 60mbtuh respectively"
Sounds like 2 nominations for the Triangle Tube Solo-60.
Thanks for including suggestions for indirect hot water too.

This morning I received a quote from contractor I provided all my measurements to: Navien CH210 or Prestige PE110.
He noted in email that he would not share heat loss analysis unless awarded the contract.

Glad I got advice here first.

Now I'll take Dana's advice and yours and do my homework on finding a contractor.

Other contractors have come in since my last post recommending oversized boilers.

But, now I have one contractor who is recommending an HTP Elite 80 (16k-80K) with T-33 indirect water tank (I believe Turbomax). He is writing up proposal which may include option for tekmar controlers.

A "reverse indirect" like a Turbomax plumbed as a massive hydraulic separator would allow you to micro-zone the place, but puts a lower bound on the water temperature, that might cut into your condensing efficiency a bit. With tall convectors like yours you can run system temps lower than domestic hot water temps and get 95%+ efficiency out of a mod-con, whereas if you need 110F water out of the thing you need to maintain the tank at 130F min, which means your return water would likely be 120F or above except after big domestic hot water draws, which would likely put you in the 92-93% efficiency range for an average.

It's almost always going to be worth it if you are going to make a bunch of tiny zones, but if the heating is still to be operated as a single zone, a standard indirect operated as it's own zone (rather than as the system's buffer tank & hydraulic separator) may be modestly more efficient. You'd have to do the real math on the real system to be sure where the boundaries are, and it's probably at most about a 3% net efficiency difference. If the guy seems otherwise competent, let him design it- just be aware that the system architecture matters.

It's almost always going to be worth it if you are going to make a bunch of tiny zones

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Maybe this explains it: He suggested I may want to consider Thermostatic Radiator Valves for each of the 13 radiators which would enable me to adjust heat to individual rooms. On first hearing this it sounded good but given the estimated cost and my budget, I said no but maybe down the road (but maybe never too).